In the packaged power industry, there is an ever increasing emphasis upon the development of high capacity, small volume electric cells. The following is a comparison of the capacity and voltage for some active materials currently used in commercially available cells.
______________________________________ Active EMF vs. Zn in Capacity Material Alkaline Electrolyte ma-hr./g amp-hr./cc ______________________________________ HgO 1.35 v. 248 2.76 Ag.sub.2 O 1.60 v. 232 1.76 AgO 1.82 v. 432 3.22 ______________________________________
Divalent silver oxide (AgO) is an excellent high capacity battery active material, but it has two properties which have limited its use as a battery active material. During the discharge of a battery employing a divalent silver oxide positive active material, the initial voltage is at the higher divalent voltage level (1.82v. vs. Zn in alkaline electrolyte) until substantially all of the AgO is converted to Ag.sub.2 O, and thereafter, the discharge continues at the lower monovalent voltage level (1.60v. vs. Zn in alkaline electrolyte). This two plateau voltage level during discharge cannot be tolerated by many types of battery operated equipment.
Another problem encountered when using divalent silver oxide as the depolarizer (positive active material) is its lack of stability when in contact with aqueous alkaline solutions. It is well known that divalent silver oxide evolves oxygen when in contact with aqueous alkaline solutions, and this gassing phenomenon causes self-discharge of the divalent silver oxide, converting it to monovalent silver oxide or metallic silver. Divalent silver oxide cannot be used as the positive active material in hermetically sealed cells because of this instability in alkaline solutions and the consequent hazard of pressure build-up and possible cell rupture.
The problem of the two plateau voltage level during the electrical discharge of divalent silver oxide has previously been overcome by the inventions disclosed in U.S. Pat. Nos. 3,615,858 and 3,655,450 issued to Luis Soto-Krebs. These patents disclose a battery having a positive electrode comprising a principal active material (e.g. divalent silver oxide) and a secondary active material (e.g. monovalent silver oxide) whose discharge product is readily oxidized by the principal active material in the presence of alkaline electrolyte, and wherein the sole electronic path of discharge of the principal active material is through the secondary active material. The battery invented by Soto-Krebs is characterized throughout discharge by the potential of the secondary active material (Ag.sub.2 O) vs. the negative electrode in the alkaline electrolyte. The battery has the advantage of a single voltage level during electrical discharge and also the increased capacity provided by the divalent silver oxide positive active material.
The problem of the divalent silver oxide instability has been overcome by the invention disclosed in U.S. Pat. Nos. 3,476,610 and 3,484,295 issued to Luis Soto-Krebs and Robert Dawson. These patents disclose a battery having a positive electrode comprising a principal active material (e.g. divalent silver oxide) and a secondary active material (e.g. monovalent silver oxide) employed as a substantially electrolyte impermeable layer interposed between the principal active material and the battery components containing the electrolyte. This construction isolates the principal active material from contact with the electrolyte until the secondary active material is discharged, thereby providing improved stand or shelf life.
There are two patents which disclose methods for treating divalent silver oxide to provide alkaline batteries having a single voltage plateau during discharge. U.S. Pat. No. 3,055,964 issued to Frank Solomon and Kenneth Brown, discloses a process for treating an oxidized silver electrode containing argentic (divalent) and argentous (monovalent) silver oxide which comprises heating the electrode to at least 50.degree. C., and preferably 50.degree. C. to 300.degree. C., for 0.003 to 1000 hours. A treatment at 100.degree. C. requires 1 hour and lower temperatures require substantially greater time. This high temperature, long duration treatment does provide a silver oxide electrode containing divalent silver oxide having a monovalent silver oxide potential during discharge.
German Pat. No. 1,496,361, issued to Yardney International Corp., also discloses a process for treating silver oxide electrodes containing divalent silver oxide for the purpose of providing alkaline batteries having a single voltage plateau during discharge. The process disclosed in the German patent comprises treating the silver oxide electrode with an aqueous silver nitrate solution to deposit a thin film of silver nitrate on the surface. Upon subsequent contact with alkaline electrolyte, a layer of monovalent silver oxide is formed on the surface of the electrode. The treatment with the silver nitrate solution requires up to an hour, with 5 to 10 minutes being sufficient if the solution is heated.